As a fixing device used in an image forming apparatus such as an electro-photographic copier or printer, there is a fixing device for inserting a sheet of paper through a nipping section formed between a pair of rollers composed of a heat roller and a pressure roller or between similar belts and heating, pressurizing, and fixing a toner image. As such a heating type fixing device, conventionally, there is an apparatus for heating a metallic conductive layer on the surface of a heat roller or a heat belt by the induction heating method. The induction heating method supplies predetermined power to an induction heating coil to generate a magnetic field, instantaneously heats the metallic conductive layer by an eddy current generated by the magnetic field, and can fix the heat roller or heat belt.
As such a fixing device of the induction heating method, for example, in Japanese Patent Application Publication No. 2002-237377, to prevent the frequency of the induction heating coil from reducing to an audio frequency of 20 kHz or less, an apparatus for retaining a fixed heating output without generating a noise is disclosed. However, this fixing device of the conventional induction heating method uses a low frequency not reaching the audio frequency to realize cut-down of cost of the apparatus.
On the other hand, in recent years, in a fixing device of the induction heating method, a fixing device for installing a thinned metallic conductive layer having a small heat capacity on the surface of a heat roller to realize fast heating of the metallic conductive layer and realizing more energy conservation has been developed. In such a thinned metallic conductive layer with a small heat capacity, when the induction heating coil is driven at a frequency of about 20 to 40 kHz by a conventionally used inverter circuit, the surface depth of an eddy current generated in the metallic conductive layer is large, so that the leakage rate of the magnetic flux is increased, and the efficient heating is disturbed.
Therefore, in a fixing device having an installed thinned metallic conductive layer, development of a fixing device of an image forming apparatus characterized in that when heating the metallic conductive layer, the magnetic flux of an eddy current generated in the metallic conductive layer by the induction heating coil leaks little, and heat generation of the metallic conductive layer is efficient, and speeded fixing and energy conservation can be realized, and the profitability and environmental maintenance are excellent is desired.